The present invention relates to apparatus for measuring radiation from a sample and particularly relates to radiation collecting and illuminating optics and a flow-through sample cell and mount therefor for a radiation detecting device, specifically a fluorescent liquid chromatography detector.
In radiation measuring apparatus of conventional construction, radiation is applied to a sample and the emission spectrum of the light from the sample is observed. Particularly, a first monochromator is utilized to disperse radiation from a light source and direct the radiation through an exit slit to the sample. An optical system is interposed to form the image of the monochromator exit slit in the sample. An additional optical system is utilized to direct the radiation from the sample to a second monochromator having entrance and exit slits, the latter optical system imaging the radiation from the sample at the entrance slit of the second monochromator. The second monochromator disperses radiation from the sample and directs it through an exit slit for detection by a detector and subsequent analysis.
In conventional instruments of the foregoing described type, the sample, which is subject to analysis, is disposed in a sample cell and is usually analyzed under static conditions. Attachments, however, have been provided for performing measurements on flowing samples. In both cases, images of the exit and entrance slits of the excitation and emission monochromators are focused in the sample cell.
In these conventional fluorescence spectrophotometers, lenses are normally utilized as the optics for directing the exit slit image of the first monochromator to the sample and the radiation from the sample to the entrance slit of the second monochromator for measurement. As is well known, lenses are subject to chromatic aberration. That is, their focus shifts when the lens transmits radiation of differing wavelengths. Thus, the excitation exit slit image and the radiation from the sample are focused at different points respectively in the sample cell and the emission entrance slit depending upon the wavelength of the radiation transmitted by the lenses. This is undesirable because resolution of the images at their design points is not maintained constant. Further, lenses diminish the energy throughput of the system due to reflection loss. This is significant because it is desirable to maximize the intensity of the light. Also, suitable lenses such as fused silica lenses are expensive to fabricate.
Further, it is desirable to utilize a small sample cell volume because it is more efficient and, in liquid chromatography applications, provides greater resolution. However, the use of small samples aggravates the aforementioned focusing problem inherent with lenses. In addition, lenses used in conjunction with very small sample cells, are increasingly expensive to fabricate because of their size and f number.